Tensile And Wear-resisting Properties Of Bimodal Ti-6Al-4V Alloy Prepared By High Energy Ball Milling And Spark Plasma Sintering

Titanium alloy is an important metallic material which is widely used not only in aircraftindustry but also in the field of civil industry due to its decreasing cost of manufacturing.With the rapid development of modern industries, the requirement of the comprehensivemechanical properties of the titanium alloy is increasingly higher. To produce Ti-6Al-4V alloywith high strength and good ductility, we proposed to prepare fine-grained Ti-6Al-4V bulkmaterial with a bimodal grain size distribution by high-energy ball-milling and subsequentspark plasma sintering (SPS) in this paper. Research was focused on the following contents:High-energy ball-milling was used to prepare nanocrystalline Ti-6Al-4V powders. Thepowders milled for6h exhibited bimodal microstructures consisting of coarse-grainedbasketweave regions and fine-grained equiaxed regions as a result of strain inhomogeneitywithin the milled powders. TEM results showed that the grain size of Ti-6Al-4V powdersball-milled for10h ranged from several tens to hundreds of nanometers.The effects of sintering temperature and the content of unmilled powders on themicrostructure and mechanical properties of sintered samples were studied. With increasingsintering temperature, the relative densities of sintered samples increased, the equiaxed finegrains tended to grow up gradually, and the tensile strengths increased first and decreasedafterwards. Moderate addition of unmilled powders into ball milled powders can cause anincrease of the coarse-grained volume fraction, which led to an enhanced ductility withoutmuch decrease in strength. When the blended powders (20wt.%unmilled) sintered at850℃for4mins, the sintered sample with relative density of99.0%and the grain sizes of1~2μmin fine-grained regions could be obtained, whose tensile yield strength, fracture strength andductility were1012MPa,1056MPa and10%, respectively.The effects of ball-milling time on the microstructure and mechanical properties ofsintered samples were researched. Results showed that specimens sintered with6h ball-milledpowders showed a hybrid microstructure which consisted of a shell structure with fine grainsand a core structure with coarse grains. Samples sintered with powders ball-milled for10hshowed bimodal ultrafine-grained structure. With increasing of ball-milling time, the average grain size gradually decreased. The tensile strength increased markedly, while the plasticstrain to failure decreased dramatically.The tribological behavior of the sintered samples with different sintering parametersunder dry sliding condition was investigated. Experimental results showed that the frictioncoefficient increased in the initial stage and then became stable with the increase of wearingtime. The friction coefficient of the sintered alloys first decreased and then increased as thesintering temperature increased. The friction coefficient and the wear volume of the sampleswith6h ball-milled powders reached the minimum values when the sintering temperature was850℃. The worn surface of the dry sliding samples presented three regions with differentdominant wear mechanisms: the adhesion wear region in the center of the wear scar, abrasivewear region in the edge areas and the plastic deformation region in the remaining areas. Whenthe volume fraction of unmilled powders increased, the friction coefficient and wear volumeof sintered compacts increased. The domimant wear mechanism of the sintered materialschanged gradually from abrasive wear to adhesion wear.The influence of grain size on the friction properties of sintered samples was also studied.The wear-resistent properties of the sintered samples with ball-milled powders were betterthan those of alloys sintered with unmilled powders in dry sliding condition. With theincreasing of ball milling time, the wear volume and the area of the wear scar were reduced.For sintered materials with unmilled powders, the wear mechanism was mainly adhesion wear,while the wear mechanism of sintered materials with ball-milled powders was primarilyabrasive and adhesion wear.